I've recently played through two different and very good Mario platformers, Mario 3 and NSMBU. I noticed that even though 3's levels were usually quite short, I died far more in that game than I did in NSMB, even though I was actively exploring the latter's levels far more in an attempt to collect all the Star Coins. There are a few reasons for this, of course, but I think the main culprit is that I can be far less precise in my platforming while playing NSMB because of the wall cling and wall jump: there's a huge built-in fudge factor which means you no longer have to nail every jump, but simply be "close enough."
To be honest, I don't like it. I've noticed when playing with friends and family that few of them take advantage of these abilities, so it's not doing them any favors, while I tend to abuse them and thereby make the game much less challenging. I believe future 2D Mario platformers should do away with the wall slide and wall jump, in the hope that this makes future games better emphasize tighter level design. Am I alone here, or do others think the same?